1. Field of the Invention
This invention relates to a drill and a method for making a pole in the ground.
2. Description of the Related Art
Most building constructions which are built on ground with compressible upper layers are erected on foundation poles. The foundation poles penetrate through the compressible upper ground layers until deep enough into a sufficiently thick hard ground layer.
The resistance of the ground against the sinking of a pole with a certain diameter rapidly increases with the depth to which the pole has penetrated the good ground. The maximal resistance is reached at a depth in good ground of approximately four times the diameter of the pole.
On the basis of results of ground research and the load which must be supported by a pole, the most economic pole length and pole diameter can be determined.
However, this calculation is only reliable if the resistance of the fixed bearing-power in the ground layer is not reduced during the making of the pole.
This requirement is met in case of ram piles because the ground is displaced where the pole enters. However, the ramming causes vibrations in the ground and knocking sounds, both of which are a problem for the surroundings.
The problems increase as the poles' diameters increase and the poles have to be driven deeper into the hard ground. As a result, the maximal diameter of a pile is limited.
Especially for making poles with a relatively large diameter, first a hole is made in the ground by means of a drill and then during the removal of this drill a hardening material such as concrete is poured in the liberated space in the drill hole.
Especially two kinds of drills are used: screw drills and displacement drills.
A screw drill or auger consists of a thin drill tube which is provided along its entire length with a screw blade having constant pitch and diameter. The drill tube is closed at the bottom by a lost tip.
This screw drill is screwed in the ground while under a downward pressure. When the desired depth is reached, concrete is pumped in the drill tube while the screw drill is retracted from the ground, mostly without rotation. The lost tip remains in the ground. The concrete fills the hole under the screw drill.
During the drilling in of the screw drill, the surface of the screw blade which is in the ground increases. Since ground pressure also increases, the friction resistance against the drilling in by a penetration per revolution equal to the pitch increases by the square of the depth. The continuous screw blade rapidly becomes unable to penetrate the ground by the pitch per revolution. As a result, a crevice develops between the top of the ground above a winding of the screw blade and the bottom of the winding above it.
This crevice, which extends in the shape of a screw along the entire drilling depth, is filled with air under atmospheric pressure. This causes the surrounding ground to be eased during the drilling in, thus decreasing the resistance against the penetration of the screw drill in the ground. However, this is very detrimental to the bearing-power of the pole.
After the screw drill is removed from the ground, the ground material which remains between the windings of the screw blade is removed. This material needs to be carried off, which is also a problem.
Displacement drills allow making the drill hole without removing ground material. Such drills contain a hollow tube which is closed at the bottom by a lost tip. The hollow tube is surrounded by a drill head which thickens in the shape of a spiral upwardly and subsequently narrows in the shape of a spiral. The drill head is moreover provided on the broadest part with a screw blade.
During the drilling into the ground, the drilling machine exerts a downward pressure on the drill tube. The drill blade also exerts a downward pressure on the drill if the penetration per revolution is smaller than the pitch of the drill blade.
The bottom of the drill head then assures a sideways displacement of the ground and, at least in compressible ground, the drill sinks per revolution by little less or even more than the pitch of the screw blade. The compressed ground then forms a casing which temporarily protects the drill hole from collapse.
In solid, difficult to compress ground, however, an empty space can develop at the bottom of the screw blade, since the sinking per revolution is considerably smaller than the pitch of the screw blade. At this empty space the ground is eased and the bearing-power of the pole is decreased.
During the screwing out, the lost tip remains in the ground and concrete is poured through the drill tube and the drill into the space being liberated under the drill. The ground which has fallen around the drill tube and the ground which has been brought by the screw blade from under the drill to this place around the drill head is again displaced by the upper part of the drill head.
Thereby it is possible that in solid ground the drill only moves up much less than the pitch of the screw blade per revolution so that a volume of ground is transported down.
This ground is then pushed in the poured concrete so that the effective diameter of the pole decreases and hence its bearing-power decreases.
This last disadvantage is even more dangerous since it occurs imperceptibly and no inspection is possible in this respect.